Multimeters are multi-function electronic instruments that typically measure such electrical parameters as dc voltage, ac voltage, current, and resistance, sometimes other physical quantities such as temperature. Most multimeters are designed to be handheld, and therefore are usually light weigh, battery powered, designed to be safe without the benefit of an earth ground connection.
Many multimeters employ "banana"-type connectors and receptacles for test lead attachment. When two banana plugs and the receptacles that are to receive them are mechanically paired and placed a standard distance apart, connection to pairs of appropriate input jacks is made quick and easy, even for an operator who is using only one hand.
In recent years, there has been some movement toward the use of connectors that make two independent electrical connections through a jack and plug that share a single central axis. Each such connector has two conductive contact areas that are electrically isolated from each other. "Old telephone" and "split banana" connectors both have this property. Split banana connectors, which are the most common in digital multimeters, are currently available in at least two versions. One version has two circumferential radially symmetrical connective regions at different distances along a central axis, with those conductive regions electrically isolated from each other by a radial band of insulative material. The other version has bilateral symmetry, with a longitudinal insulator separating two longitudinal conductive regions. Both types of these connectors are described and illustrated in U.S. Pat. No. 5,508,621 to Wong for a "Four-Terminal Ohmmeter Apparatus", hereby incorporated by reference.
To differentiate them from coaxial cables and other such connectors that have two conductive parts but wherein one of those conductive parts is dedicated to being exclusively used as a ground reference and electrical and/or mechanical shielding, we will hereinafter refer to this class of connectors as "dual-signal, single-axis" connectors.
One of the desirable features of these dual-signal, single-axis connectors is that they can mate and work effectively with their single-signal, single-axis counterparts, and thereby provide "backward compatibility". Moreover, when used with their single-signal counterparts, these dual-signal connectors have their separate conductive regions brought into electrical contact with each other by the conductive part of their single-signal counterpart. In some applications, this can be taken advantage of by detecting when the two parts that are normally insulated from each other become shorted together. That information can be monitored and provide an indication that a particular jack is in use. Along with other information known to the instrument's software, that particular piece of information might be informative of how the instrument is presently being used and therefore which functions or safety features should be made active.
While some larger multimeters and small bench top instruments have communication ports that allow them to be controlled by a computer over a general purpose interface bus (GPIB) or other similar type of communication bus, most handheld multimeters do not provide facilities for computer communication, or provide for such communication via a dedicated multi-pin port, such as a RS-232, GBIP, PS/2, or some other serial or parallel port. There are serious safety concerns with any wire-connected communication ports, however. Because handheld instruments may be connected to input voltages as large as 1,000 V, to be safe, communication ports require electrical isolation, for example, by optical coupling. An optical coupling system adds to size, weight, and expense. Since these factors are all at a premium in handheld instruments, such instruments have usually been designed to forego having a multi-pin communication port for calibration or control by a computer. Nonetheless, having a facility for communication with a computer is highly desirable, both for communication of calibration data and for communication of operational control signals and measurement results data.